2 About the ITU BS.1770 Loudness Measurement
The ITU Reco endation BS.1770 specifies the algorith s to easure audio
progra e loudness. The RLB-LU application i ple ents these algorith s to
display the loudness of a Stereo signal as well as for a Surround Sound Signal.
(The MSD100C-LOUDNESS can only easure Loudness for a stereo signal)
Different working odes of the eter are selectable, including:
· Fast ode for real ti e control of loudness.
· Integrating ode for easuring the loudness of a recorded section or the
co plete recording.
2.1 Applications
There are any applications, where it is necessary to easure and control the
perceived loudness of audio signals.
Exa ples of this includes television and radio broadcast applications, where the
nature and content of the progra e aterial changes frequently. In these
applications the audio content can continually switch between usic, speech
and sound effects. Such changes in the content of the progra e aterial can
result in significant changes in subjective loudness.
Moreover, various for s of dyna ic processing are frequently applied to the
signals, which can have significant effect on the perceived loudness of the
signal.
The atter of subjective loudness is also of great i portance to the usic
industry and in the production of co ercials, where dyna ic range processing
is co only used to axi ize the perceived loudness.
2.2 Loudness Measurements
It is thus well recognized that loudness etering is required for broadcast
applications.
The algorith reco ended by ITU and i ple ented in the RLB-LU application
is based on an extension of the Leq(RLB) algorith to cover stereo and
ultichannel audio signals. In this algorith the Leq is an unweighted ean-
square easure, while the RLB involves a high pass frequency weighting curve
referred to as the Revised Low-frequency B-curve.
Loudness is a perceptual property of an audio signal, when it is reproduced
acoustically. It is a co plex non-linear function of a plitude, frequency and
bandwidth.
Present audio level eters easure the level of audio signals expressed as the
a plitude of the signal - either the r s voltage of an electrical signal or the
sound pressure of an acoustic signal.
This level is an objective property, which is independent of frequency and
bandwidth and is easured linearly in volts, if electrical, or Pascals, if acoustic.
For the purpose of broadcasting, loudness can also be easured as an
electrical property, assu ing a fixed electro-acoustic gain for reproduction. This
assu ption is the basis for the broadcast loudness eter. The reproduction
level, that has been assu ed in the ho e, is 60 dBA. A level found to be typical
for television viewing at ho e.
The RLB-LU application has two ain operating odes for different purposes:
Fast ode (SLIDE) for instantaneous onitoring of the loudness level. This
ode is used in production, post-production and presentation. The progra e
level should be set so that on typical dialogue the eter displays 0 LU on
average.
Integrating ode for a single figure overall loudness. This ode is used for
quality control, ainly at progra e ingestion, progra e e ission and in
post- orte analysis. The single figure output fro this ode allows clear and
una biguous infor ation for loudness atching and gain setting. This ode
akes it easy to secure consistent loudness over different progra es and
short sequences.
